[SyntaxModel] Remove some dead code

This code doesn't quite do what it says it does,
and appears to be dead as we should never form an
argument list TupleExpr with an OptionalEvaluationExpr
parent. Given the test case that was added with it
still passes, let's remove it.

Author: hamishknight

lib/IDE/SyntaxModel.cpp

@@ -571,12 +571,6 @@ std::pair<bool, Expr *> ModelASTWalker::walkToExprPre(Expr *E) {
   if (auto *ParentTupleExpr = dyn_cast_or_null<TupleExpr>(Parent.getAsExpr())) {
     // the argument value is a tuple expression already, we can just extract it
     addCallArgExpr(E, ParentTupleExpr);
-  } else if (auto *ParentOptionalExpr = dyn_cast_or_null<OptionalEvaluationExpr>(Parent.getAsExpr())) {
-    // if an argument value is an optional expression, we should extract the
-    // argument from the subexpression
-    if (auto *ParentTupleExpr = dyn_cast_or_null<TupleExpr>(ParentOptionalExpr->getSubExpr())) {
-      addCallArgExpr(E, ParentTupleExpr);
-    }
   }
 
   if (E->isImplicit())